Did you try increasing the rpm's to see if there was a major change in amplitude?

My thinking in this is if possible to detect slip ring/brush noise or rectifier break down. There appears to be a lot of noise in the negative side of the cycle at both the battery and especially the radio.

Consider that alternator circuits are a simple bridge rectifier with very little filtering. It is likely the radio head manufacturer has some filtering for the input power but offhand we usually don't know its limits. When the alternator rectifier has a problem, it can pass on a lot of nasty spikes because of the lack of filtering.

Have you done any additional grounding at the alternator, head unit and amplifiers? Do they have a common connection on the frame?

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )